[Bf-cycles] sample with lamp changes light paths- normal behavior?

[Brecht Van Lommel]

If the material does not contain an emissive shader, the option does
nothing. Ideally we should gray this out in such cases but it's slow
to check this when drawing the UI.

Brecht.

On Tue, Nov 13, 2012 at 11:07 PM, Matthew Heimlich
<matt.heimlich at gmail.com> wrote:
> Are there any inherent pros/cons of leaving sample as lamp on for every
> material? If there are cons, why is it enabled by default and not just when
> you make an emissive material?
>
> On Nov 13, 2012 4:54 PM, "Brecht Van Lommel" <brechtvanlommel at pandora.be>
> wrote:
>>
>> I would not call this a bug, not because it's great behavior but
>> because it's part of an important optimization.
>>
>> When sample as lamp is disabled, what happens is that rays just bounce
>> around based on the BSDF, and when they happen to hit a an emitter,
>> then that illuminates the surface. In this case we don't know
>> beforehand that we are going to hit an emitting surface, and so we
>> don't know if it's a "shadow ray".
>>
>> With samples as lamp enabled, that same type of bouncing still happen.
>> But in addition we now pick a points on emitting surfaces and trace
>> shadow rays directly to it, which helps finding small emitting
>> surfaces. In this case we know it's a shadow ray, and so the ray is
>> tagged as such.
>>
>> The solution would involve executing shaders twice to distinguish
>> between these two cases, so not ideal.
>>
>> Brecht.
>>
>> On Tue, Nov 13, 2012 at 10:01 PM, Bassam Kurdali <bassam at urchn.org> wrote:
>> > Hey folks, I've constructed an artificial example here to illustrate the
>> > issue.
>> > I've got a plane (named selective) that has transparent shader and a mix
>> > node with Light Path 'Is Shadow Ray' as input to factor. The idea is
>> > that the shadows would be colored one color, but e.g. diffuse and glossy
>> > reflections with another.
>> >
>> > This is being lit with a mesh cube and an emission shader.
>> >
>> > If 'Sample as lamp' is on on the Cube emission shader, the result is a
>> > yellow transparent plane, with a blue shadow.
>> >
>> > If 'Sample as lamp' is off on the Cube emission shader, the result is a
>> > yellow transparent plane, with a yellow shadow.
>> >
>> > Is this quirk a normal result? I realize that transparent shader is
>> > somehow 'fake' - or is it a bug?
>> >
>> > blend file http://urchn.org/misc/test.blend
>> > image1 http://urchn.org/misc/sample_aslamp.png
>> > image2 http://urchn.org/misc/nosampleaslamp.png
>> >
>> > cheers
>> > Bassam
>> >
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